Field of the Invention
This invention relates in general to methods and systems for optical inspection of electronic devices, such as LCD and OLED displays, and, more particularly, to providing systems and methods for viewing through the optical thin film color filters and their overlaps.
Description of the Related Art
Liquid crystal display (LCD) panels incorporate liquid crystals that exhibit electric-field dependent light modulating properties. They are used most frequently to display images and other information in a variety of devices ranging from fax machines, cell phones, tablet and laptop computer screens, to large screen, high-definition TVs. Active matrix LCD panels are complex layered structures consisting of several functional layers: one or more layers of polarizing film; a TFT glass substrate incorporating thin-film transistors, storage capacitors, pixel electrodes and interconnect wiring, a color filter glass substrate incorporating a black matrix and a color filter array and a transparent common electrode; an orientation film made of polyimide; and the actual liquid crystal material incorporating plastic/glass spacers to maintain proper LCD cell thickness.
A novel color-filter-on-array (COA) technology permits color filters and TFT array to be fabricated on the same glass panel reducing the overall cost and improving viewing angle characteristics of the LCD device. Pursuant to this technology, the color filters are deposited directly on top of thin-film transistors formed on the TFT glass substrate. However, the deposited color filter material itself as well as various imperfections of the color filter deposition process, cause difficulty in viewing features buried underneath the aforesaid color filters and their overlaps during the inspection phase of the LCD manufacturing process.
Specifically, viewing through thin films to resolve features buried underneath presents a challenge due to interference effects which result in fringes and other artifacts appearing in captured image and making resolving of these features difficult or impossible. This task becomes further complicated due to the following circumstances. First, thin film is a color filter having optical transmission window which is just a fraction of illumination spectrum. Second, different color filters have transmission windows in different wavelength regions of optical spectrum and each single captured image contains spatial regions covered by different color filters. Third, there are spatial regions, boundaries, where different color filters spatially overlap being deposited on top of one another. Fourth, the thickness of the color filter film within the filter boundary may vary substantially. Finally, image acquisition must be done under control of machine vision algorithm and on the fly which requires high brightness source of illumination to generate enough light during exposure time or make a strobe pulse which is sufficiently short to prevent image blur.
Conventional solutions to the above problems involve using powerful broadband illumination light sources and filtering a small portion of the light from the object in the near infrared (IR) spectrum. At wavelengths in the infrared (IR) spectrum, the color filter materials have comparable optical transmissivity values, which results in substantial elimination of many of the aforesaid undesirable effects and greatly improves the visibility of underlying object structures.
However, the powerful broadband illumination sources used in conventional approaches generate substantial amounts of heat and do not produce sufficient light intensity to allow strobe imaging used for rapid image acquisition in modern high-throughput electronic device inspection systems.